Do you have a rooster or one close by? Hens are notorious for being sneaky and hiding nests in the most unlikely places cause they decided to go broody. My flock is enablers if one goes broody they will help add to nest cause everyone seems to think they should try and hatch 30 eggs so they are gonna help her even though the most they have hatched out is 10.
time of year age and breed needs to be taken into consideration , some are better winter layers and some pretty much stop once it gets cold and less light, it's their rest period. Molting usually drops a week or 2 before they molt, they they are off for a bit after they finish as making them feathers takes much of the same nutritional resources as making an egg.
External or internal parasites are another possibility. are their poops looking ok? Have you checked for anything externally?
Reproductive issue is another possibility, egg bound or laying internally.
not that common but being plugged up can cause issues too as both come out the vent.